Timing detection method, communication system and receiving apparatus

This application is a 371 U.S. National Phase of International Application No. PCT/JP2022/019061, filed on Apr. 27, 2022. The entire disclosure of the above application is incorporated herein by reference.

The present invention relates to a timing detection method, a communication system, and a receiving apparatus.

In a communication system that performs transmission and reception in units of packets, it is necessary to accurately find a head position of a physical frame in order to perform synchronization processing. However, since there is impulse noise underwater, the timing of the impulse noise is often erroneously detected. In order to prevent erroneous timing detection in an environment with impulse noise, a relatively long correlation sequence may be embedded in a received signal, and frame timing detection may be performed by cross-correlation (e.g., refer to Non Patent Literature 1).

Non Patent Literature 1: M. Stojanovic, J. A. Catipovic and J. G. Proakis, “Phase-coherent digital communications for underwater acoustic channels,” in IEEE Journal of Oceanic Engineering, vol. 19, no. 1, pp. 100-111, January 1994, doi: 10.1109/48.289455

The possibility of falsely detecting the impulse noise as the frame timing decreases as the correlation sequence becomes longer. On the other hand, the cross-correlation value between the Doppler-shifted received signal and the correlation sequence decreases as the correlation sequence becomes longer.

6 FIG. 6 FIG. 6 FIG. 11 12 21 22 is a diagram illustrating the detection performance of a conventional technology.illustrates, as detection performance, a relationship between a moving speed and a cross-correlation value at the time of physical frame reception detection in the conventional technology. As the cross-correlation value, a normalized signal to noise ratio (SNR) is used. Reference sign L denotes a detectable level. When the cross-correlation value exceeds the detectable level L, the reception of the physical frame can be detected. Reference sign Gdenotes a cross-correlation value in a case where the sequence length is 25 ms, and reference sign Gdenotes a detectable range in a case where the sequence length is 25 ms. Reference sign Gdenotes a cross-correlation value in a case where the sequence length is 100 ms, and reference sign Gdenotes a detectable range in a case where the sequence length is 100 ms. As illustrated in, the cross-correlation value around the moving speed of 0 m/s becomes larger and the SNR is more improved as the sequence length becomes longer. This indicates that resistance to impulse noise is increased. On the other hand, the range of the moving speed in which the reception of the physical frame can be detected becomes narrower as the sequence length becomes longer.

In this way, it becomes difficult to detect the timing in a mobile environment if the correlation sequence is lengthened in order to improve the impulse noise resistance.

In view of the above circumstances, an object of the present invention is to provide a timing detection method, a communication system, and a receiving apparatus that improve the detection accuracy of a received signal in a mobile environment while preventing decrease in impulse noise resistance.

A timing detection method according to an aspect of the present invention includes: a transmission step of intermittently transmitting a physical frame of a transmitted signal by a transmission apparatus; a correlation calculation step of calculating, by a receiving apparatus, a correlation between a received signal and each of correlation sequences subjected to Doppler shift of a plurality of types of shift amounts; and a determination step of determining, by the receiving apparatus, that the physical frame has been received in a case where any one of calculation results obtained in the correlation calculation step exceeds a threshold.

A timing detection method according to an aspect of the present invention includes: a correlation calculation step of calculating a correlation between a received signal and each of correlation sequences subjected to Doppler shift of a plurality of types of shift amounts; and a determination step of determining that a physical frame transmitted from a transmission apparatus that intermittently transmits a physical frame of a transmitted signal has been received in a case where any one of calculation results obtained in the correlation calculation step exceeds a threshold.

An aspect of the present invention is a communication system including a transmission apparatus and a receiving apparatus, in which the transmission apparatus intermittently transmits a signal frame, and the receiving apparatus includes: a correlation calculation unit (hereinafter also referred to as “correlation calculation circuitry”) that calculates a correlation between a received signal and each of correlation sequences subjected to Doppler shift of a plurality of types of shift amounts; and a determination unit (hereinafter also referred to as “determination circuitry”) that determines that a physical frame has been received in a case where any one of calculation results obtained by the correlation calculation unit exceeds a threshold.

A receiving apparatus according to an aspect of the present invention includes: a correlation calculation unit that calculates a correlation between a received signal and each of correlation sequences subjected to Doppler shift of a plurality of types of shift amounts; and a determination unit that determines that a physical frame transmitted from a transmission apparatus that intermittently transmits a physical frame of a transmitted signal has been received in a case where any one of calculation results obtained by the correlation calculation unit exceeds a threshold.

According to the present invention, it is possible to improve the detection accuracy of a received signal in a mobile environment while preventing decrease in impulse noise resistance.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

A communication system according to the present embodiment improves the detection accuracy of physical frame reception in a mobile environment while preventing decrease in impulse noise resistance without shortening the correlation sequence. As a result, the communication system according to the present embodiment accurately detects the reception timing of the physical frame of the signal even in an environment where impulse noise exists.

1 FIG. 1 1 10 20 1 20 10 is a diagram illustrating a configuration of a communication systemaccording to an embodiment of the present invention. The communication systemincludes a transmitterand a receiver. The communication systemis, for example, an underwater communication system that performs underwater communication. In the case of performing underwater communication, the receiverreceives a signal transmitted underwater from the transmitter.

10 The transmitterintermittently transmits physical frames of transmitted signals from an antenna. The physical frame includes a payload and a correlation sequence. The correlation sequence is set at a predetermined position of the physical frame. In the present embodiment, a correlation sequence is set at the head of a physical frame. The correlation sequence is known.

20 10 20 21 22 23 The receiverintermittently receives, by an antenna, the physical frames intermittently transmitted from the transmitter. The receiverincludes a timing detection device, a demodulation device, and a shift amount calculation device.

21 21 22 The timing detection devicecalculates a correlation between the received signal received by the antenna and each of correlation sequences subjected to Doppler shift of a plurality of types of Doppler shift amounts, and detects the reception of the physical frame on the basis of the calculation results. The timing detection devicenotifies the demodulation deviceof the received signal and the detection position of the physical frame in the received signal.

22 21 21 22 10 The demodulation devicespecifies the position of the payload in the received signal on the basis of the detection position of the physical frame detected by the timing detection device. In a case where the correlation sequence is included at the head of the physical frame, the detection position in notification from the timing detection devicerepresents the head of the physical frame in the received signal. The demodulation devicedemodulates a portion of the received signal corresponding to the payload of the physical frame to obtain the information transmitted by the transmitter.

23 21 23 20 20 23 21 The shift amount calculation devicecalculates a plurality of types of Doppler shift amounts to be applied to the correlation sequence used by the timing detection device. Note that the shift amount calculation devicemay be provided outside the receiver. Moreover, the receivermay not include the shift amount calculation device. In this case, the timing detection deviceuses a plurality of predetermined types of Doppler shift amounts set in advance.

7 FIG. 91 91 911 912 911 912 Here, a conventional technology will be described.is a diagram illustrating a configuration of a conventional timing detection device. The timing detection deviceincludes a correlatorand a threshold determination unit. The correlatorcalculates a correlation value between the received signal and the correlation sequence. The threshold determination unitdetects the reception of the physical frame by comparing the correlation value with the threshold.

8 FIG. 91 911 91 911 92 912 92 93 is a flowchart illustrating reception timing detection processing by the timing detection device. The correlatorinputs a received signal (step S). The correlatorcalculates cross-correlation between the received signal and the known correlation sequence (step S). The threshold determination unitdetermines whether the correlation value of the cross-correlation calculated in step Sis larger than a threshold or not (step S).

93 912 91 93 912 912 91 94 When determining that the correlation value is smaller than or equal to the threshold (step S: NO), the threshold determination unitrepeats the processing from step Sfor a received signal at the next timing. When determining that the correlation value is larger than the threshold (step S: YES), the threshold determination unitdetects the reception of the physical frame. When the correlation sequence is at the head of the physical frame, the threshold determination unitnotifies the subsequent stage of the head position of the received signal inputted in step Sas the detection position of the physical frame (step S).

91 In the case of the timing detection device, in principle, it is more difficult to detect the Doppler-shifted received signal as the correlation sequence becomes longer.

21 21 21 211 212 213 212 212 1 212 2 FIG. Next, the timing detection deviceaccording to the present embodiment will be described.is a diagram illustrating a configuration of the timing detection device. The timing detection deviceincludes a branching unit, N (N is an integer of 2 or more) correlators, and a threshold determination unit. The N correlatorsare referred to as correlators-to-N, respectively.

211 212 1 212 212 212 1 n n n n 1 N n 1 1 n The branching unitbranches the received signal into N branches, and inputs the N-branched received signals to the correlators-to-N, respectively. A correlator-(n is an integer of 1 or more and N or less) calculates a correlation value Cbetween the received signal and a correlation sequence Aobtained by applying Doppler shift of a frequency shift amount f[Hz] on the correlation sequence A. The frequency shift amounts fto fare different values. Any one of the frequency shift amounts fmay be 0. For example, it is assumed that the frequency shift amount fof a correlator-is set to 0. In this case, the correlation sequence A is used as it is as the correlation sequence A. In the present embodiment, a normalized SNR is used as the correlation value C.

213 212 1 212 213 22 1 N The threshold determination unitdetects the reception of a physical frame in a case where any one of the correlation values Cto Ccalculated respectively by the correlators-to-N exceeds the threshold. The threshold determination unitoutputs the received signal and the detection position of the physical frame to the demodulation device.

3 FIG. 21 211 11 211 212 1 212 is a flowchart illustrating reception timing detection processing by the timing detection device. The branching unitinputs the received signal received by the antenna (step S). The branching unitbranches the received signal into N branches, and inputs the N-branched received signals to the correlators-to-N, respectively.

212 12 212 1 212 213 n n n n n 1 n The correlator-calculates a correlation value Cbetween a portion from the head of the received signal to the length of the correlation sequence A and a correlation sequence An obtained by applying f[Hz] Doppler shift on a known correlation sequence A (step S-). Here, the correlator-uses the correlation sequence A as it is as the correlation sequence A. The correlator-outputs the calculated correlation value Cto the threshold determination unit.

213 212 1 212 12 1 12 213 13 1 N 1 N The threshold determination unitinputs the correlation values Cto Ccalculated by the correlators-to-N in steps S-to S-N, respectively. The threshold determination unitdetermines whether any one of the correlation values Cto Cexceeds the threshold or not (step S).

1 N 1 N 13 213 11 13 213 213 22 11 14 When determining that all of the correlation values Cto Care equal to or less than the threshold (step S: NO), the threshold determination unitrepeats the processing from step Sfor a received signal at the next timing. When determining that any one of the correlation values Cto Cexceeds the threshold (step S: YES), the threshold determination unitdetects the reception of the physical frame. In a case where the correlation sequence is located the head of the physical frame, the threshold determination unitnotifies the demodulation deviceof the head position of the received signal inputted in step Sas the detection position of the physical frame (step S).

21 As described above, the timing detection devicecalculates the cross-correlation between the received signal and the Doppler-shifted correlation sequence in parallel in addition to the cross-correlation between the received signal and the known correlation sequence. As a result, it is possible to detect the Doppler-shifted received signal without sacrificing the detection accuracy.

23 212 1 212 21 Next, a method of calculating, by the shift amount calculation devices, the frequency shift amounts set respectively for the correlators-to-N of the timing detection devicewill be described.

In a first calculation method, a Doppler frequency that cannot be detected can be eliminated. Since the Doppler frequency changes according to the moving speed, it can be said that a moving speed at which detection is impossible can be eliminated.

d s s n d 2 2 It is assumed that M denotes a sequence length of a correlation sequence, fdenotes a Doppler frequency, Tdenotes a symbol rate, σdenotes signal power of a received signal, and σdenotes noise power of a received signal. The Doppler frequency is the amount of change between a frequency subjected to the Doppler effect and a frequency before being subjected to the Doppler effect. The signal power S(f) at the time of received signal detection is expressed by the following Expression (1).

d Expression (1) represents a value of a case where a correlation sequence having a Doppler shift amount of 0 is used. The noise power N(f) at the time of physical frame reception detection is expressed by the following Expression (2).

The SNR at the time of physical frame reception detection is calculated by the following Expression (3) using Expressions (1) and (2).

d It is assumed that the Doppler frequency fis expressed by the following Expression (4).

d d s d s d s 4 FIG. 23 212 212 1 212 In this case, the signal power is expressed as S(f)=0. That is, in a case where the Doppler frequency is expressed as f=k/(MT) [Hz], the SNR at the time of physical frame reception detection becomes 0, and thus signal detection cannot be performed.is a diagram illustrating the relation between a correlation value and a Doppler shift amount at the time of physical frame detection. At the Doppler shift amounts fa and fb, the SNR, which is a correlation value, is 0, and therefore, signal detection is impossible. Therefore, the shift amount calculation deviceshifts the Doppler frequency by f=1/(MT) [Hz] and sets the result for each correlator. That is, the correlators-to-N calculate correlation values with respect to the received signals by using correlation sequences Doppler-shifted by f=1/(MT). As a result, it is possible to eliminate the Doppler frequency at which SNR=0 is satisfied.

s s s 10 20 As described above, in the first calculation method, each of the plurality of types of Doppler shift amounts is set to any value obtained by multiplying the reciprocal 1/(MT) of the product MTof the sequence length M of the correlation sequence and the symbol rate Tof the physical frame by an integer. Note that the number of sequences M of the correlation sequence is determined according to the range of the moving speed assumed for the communication apparatus including the transmitterand the receiver, that is, the Doppler shift amount assumed from the range of the moving speed. Moreover, the detection threshold is set on the basis of a receiver operating characteristic (ROC) according to a risk rate (significance level) required by the system and detection power.

23 The shift amount calculation devicedetermines the frequency shift amount by the following procedure. According to this method, the SNR of the lower detection limit can be secured.

23 23 212 1 212 1 (Procedure 1) The shift amount calculation devicegenerates a correlation sequence A of a Doppler frequency, which is 0, and sets the correlation sequence A as a correlation sequence A1. The shift amount calculation devicesets the correlation sequence A1 for the correlator-. This corresponds to regarding one Doppler shift amount (here, 0) among a plurality of types of Doppler shift amounts as a reference and setting a correlation sequence of a reference Doppler shift amount for the correlator-.

23 23 min (Procedure 2) The shift amount calculation devicedetermines a detection threshold TH for the received signal of the Doppler frequency, which is 0, on the basis of the general ROC. The shift amount calculation deviceobtains an SNR at the detection threshold TH of the received signal with the Doppler frequency, which is 0, and sets the SNR as C.

23 d1 (Procedure 3) When receiving the received signal in which the correlation sequence is set at the head of the physical frame, the shift amount calculation deviceobtains the Doppler frequency±fthat satisfies the condition expressed in the following Expression (5).

d d d1 min That is, the ratio of the signal power S (f) and the noise power N (f) of the received signal calculated using the correlation sequence of the Doppler frequency±fmatches the ratio Cof the signal power and the noise power of the received signal at the detection threshold calculated using the correlation sequence of the reference Doppler shift amount.

23 212 2 212 3 23 212 4 212 5 23 212 d1 d1 d1 (Procedure 4) The shift amount calculation devicesets the correlation sequences A2 and A3 obtained by shifting the correlation sequence A1 by the Doppler frequency±frespectively for the correlators-and-. Furthermore, the shift amount calculation devicesets the correlation sequences A4 and A5 shifted by the Doppler frequency±2×frespectively for the correlators-and-. Hereinafter, the shift amount calculation devicerepeats up to the correlator-N. As a result, the plurality of types of shift amounts have values shifted by the Doppler frequency f.

5 FIG. 5 FIG. 21 91 10 21 212 212 1 212 2 212 3 911 91 is a diagram illustrating detection performance of the timing detection deviceobtained by computer simulation.also illustrates detection performance of the conventional timing detection devicefor comparison. In this simulation, the correlation sequence length of the physical frame transmitted from the transmitteris set to 100 ms, and the carrier frequency is set to 20 kHz. Moreover, the timing detection deviceincludes three correlators, the correlator-uses a correlation sequence shifted by 0 Hz, the correlator-uses a correlation sequence shifted by 10 Hz, and the correlator-uses a correlation sequence shifted by-10 Hz. Moreover, the correlatorof the timing detection deviceuses a correlation sequence shifted by 0 Hz. Moreover, the detectable SNR at the time of detection was set to −10 dB.

212 212 911 911 5 FIG. Reference sign L denotes a detectable level. In a case where the SNR is higher than the detectable level L, the reception of the physical frame can be detected. R11 denotes a detected SNR in the correlatorof the present embodiment, and R12 denotes a detectable range in the correlatorof the present embodiment. R21 denotes a detected SNR in a conventional correlator, and R22 denotes a detectable range in the conventional correlator. With reference to, according to the present embodiment, the range of the moving speed at which the physical frame can be detected is widened, and it can be seen that the frame detection accuracy in the mobile environment can be improved without sacrificing the sequence length.

20 20 Some of the functions of the receiverin the above-described embodiment may be implemented by a processor such as a central processing unit (CPU) or a graphics processing unit (GPU) reading and executing a program from a recording medium such as a memory. Moreover, some of the functions of the receivermay be implemented by using hardware such as an application specific integrated circuit (ASIC), a programmable logic device (PLD), or a field programmable gate array (FPGA).

10 20 212 213 According to the above-described embodiment, the communication system includes a transmission apparatus and a receiving apparatus. For example, the transmission apparatus corresponds to the transmitterin the embodiment, and the receiving apparatus corresponds to the receiverin the embodiment. The transmission apparatus intermittently transmits a physical frame of a transmitted signal. The receiving apparatus includes a correlation calculation unit and a determination unit. The correlation calculation unit corresponds to, for example, the correlatorof the embodiment. The correlation calculation unit calculates a correlation between the received signal and each of the correlation sequences subjected to Doppler shift of a plurality of types of shift amounts. The received signal may be a signal transmitted underwater. The determination unit corresponds to, for example, the threshold determination unitof the embodiment. The determination unit determines that the physical frame has been received in a case where any one of the calculation results obtained by the correlation calculation unit exceeds the threshold.

s s s Each of the plurality of types of shift amounts is any value obtained by multiplying a reciprocal (1/MT) of a product MTof the sequence length M of the correlation sequence and the symbol rate Tof the physical frame by an integer.

d1 d d d1 min Alternatively, the plurality of types of shift amounts are values shifted by a predetermined Doppler frequency f. In a case where one of the plurality of types of shift amounts is regarded as a reference, a ratio of the signal power S(f) and the noise power N(f) of the received signal calculated using the correlation sequence of the predetermined Doppler frequency fmatches an SNR (e.g., corresponding to Cof the embodiment) that is a ratio of the signal power and the noise power of the received signal at the detection threshold calculated using the correlation sequence of the reference shift amount.

Although the embodiment of the present invention has been described in detail with reference to the drawings, specific configurations are not limited to the present embodiment and include design and the like within a scope not departing from the gist of the present invention.

1 Communication system 10 Transmitter 20 Receiver 21 91 ,Timing detection device 22 Demodulation device 23 Shift amount calculation device 211 Branching unit 212 1 212 911 -to-N,Correlator 213 912 ,Threshold determination unit 911 Correlator